Adding this song, which is in the drama, but not part of OST :
The song in the end of episode 21, sung by the mom of Lu Linjia, Han Yumei (played by Tian Miao, but I'm not sure it was her who sang for real) is "Love needs practice" (爱需要练习) originally sung by Liu Ruiqi (刘瑞琦) in 2015 c-drama Mission Impossible Love 爱情碟中谍, and next recorded by Chen Ming (陈明). Nice music quote, to look up nice songs and c-dramas : you can look up the Liu Ruiqi version which has English subtitles, on YouTube. https://www.youtube.com/watch?v=CtCS7JiCrJs A nice, slow, love song.
(The drama 爱情碟中谍 is available on YouTube, but without subtitles)